The association of HL-A antigens with disease susceptibility is being studied in a large number of human diseases. The possibility that an immune response gene, linked to the histocompatibility locus, may control the reactivity of individuals to microbial antigens is being investigated. Family studies are underway to establish linkage. Cross-reactivity of microbial products with HL-A is being examined by inhibition tests. Certain diseases, such as ankylosing spondylitis, Reiter's disease and juvenile rheumatoid arthritis, have been shown to be very highly associated with the W27 antigen. The reason for this association is being sought.